Cell Structure and Function

Impacts, Issues: Food For Thought  A strain of E. coli bacteria that causes severe illness or death occasionally contaminates foods such as ground beef and fresh vegetables

4.1 The Cell Theory  The cell theory, a foundation of modern biology, states that cells are the fundamental units of life

Measuring Cells  One micrometer (μm) is one-thousandth of a millimeter

Animalcules and Beasties  Van Leeuwenhoek was the first to describe small organisms seen through a microscope, which he called animalcules and beasties  Hooke was the first to sketch and name cells

Development of the Microscope

The Cell Theory Emerges  In 1839, Schleiden and Schwann proposed the basic concepts of the modern cell theory All organisms consists of one or more cells A cell is the smallest unit with the properties of life Each new cell arises from division of another, preexisting cell Each cell passes its hereditary material to its offspring

4.2 What Is a Cell?  Cell The smallest unit that shows the properties of life  All cells have a plasma membrane and cytoplasm, and all start out life with DNA

The Basics of Cell Structure  Eukaryotic cell Cell interior is divided into functional compartments, including a nucleus  Prokaryotic cell Small, simple cells without a nucleus

All Cells Have Three Things In Common  Plasma membrane Controls substances passing in and out of the cell  DNA containing region Nucleus in eukaryotic cells Nucleoid region in prokaryotic cells  Cytoplasm A semifluid mixture containing cell components

Prokaryotic and Eukaryotic Cells

Fig. 4-4a, p. 56

Fig. 4-4b, p. 56

Cells Have Large Surface Area-to-Volume Ratio

Cell Size  Surface-to-volume ratio restricts cell size by limiting transport of nutrients and wastes

Preview of Cell Membranes  Lipid bilayer A double layer of phospholipids organized with their hydrophilic heads outwards and their hydrophobic tails inwards Many types of proteins embedded or attached to the bilayer carry out membrane functions

Basic Structure of Cell Membranes

Fig. 4-6a, p. 57 hydrophilic head two hydrophobic tails A A phospholipid, the main type of lipid in cell membranes.

Fig. 4-6b, p. 57

Fig. 4-6c, p. 57

Key Concepts: What All Cells Have In Common  Each cell has a plasma membrane, a boundary between its interior and the outside environment  The interior consist of cytoplasm and an innermost region of DNA

4.3 How Do We See Cells?  We use different types of microscopes to study different aspects of organisms, from the smallest to the largest

Modern Microscopes  Light microscopes Phase-contrast microscopes Reflected light microscopes Fluorescence microscopes  Electron microscopes Transmission electron microscopes Scanning electron microscopes

Light and Electron Microscopes

Fig. 4-7a, p. 58

Fig. 4-7b, p. 58

Fig. 4-8, p. 59 a) Light micrograph. A phase-contrast micro-scope yields high-contrast images of transparent specimens, such as cells. b) Light micrograph. A reflected light micro-scope captures light reflected from opaque specimens. c) Fluorescence micro-graph. The chlorophyll molecules in these cells emitted red light (they fluoresced) naturally. d) A transmission electron micrograph reveals fantastically detailed images of internal structures. e) A scanning electron micro- graph shows surface details of cells and structures. Often, SEMs are artificially colored to highlight certain details. Stepped Art Different Microscopes, Different Characteristics

4.3 Key Concepts: Microscopes  Microscopic analysis supports three generalizations of the cell theory: Each organism consists of one or more cells and their products A cell has a capacity for independent life Each new cell is descended from a living cell

4.4 Introducing Prokaryotic Cells  Bacteria and archaea are the prokaryotes (“before the nucleus”), the smallest and most metabolically diverse forms of life  Bacteria and archaea are similar in appearance and size, but differ in structure and metabolism

General Prokaryote Body Plan  Cell wall surrounds the plasma membrane Made of peptidoglycan (in bacteria) or proteins (in archaea) and coated with a sticky capsule  Flagellum for motion  Pili help cells move across surfaces Sex pilus aids in sexual reproduction

Fig. 4-10, p. 60 flagellum capsule cell wall plasma membrane cytoplasm, with ribosomes DNA in nucleoid pilus

Archaeans

Bacteria

4.5 Microbial Mobs  Although prokaryotes are all single-celled, few live alone  Biofilm Single-celled organisms sharing a secreted layer of polysaccharides and glycoproteins May include bacteria, algae, fungi, protists, and archaeans

A Biofilm

Key Concepts: Prokaryotic Cells  Archaeans and bacteria are prokaryotic cells, which have few, if any, internal membrane- enclosed compartments  In general, they are the smallest and structurally the simplest cells

4.6 Introducing Eukaryotic Cells  Eukaryotic (“true nucleus”) cells carry out much of their metabolism inside membrane-enclosed organelles  Organelle A structure that carries out a specialized function within a cell

Organelles of Eukaryotic Cells

Eukaryotes: Animal and Plant Cells

Fig. 4-14a, p. 62 vacuole plasma membrane mitochondrion nucleus 1 µm (a) Human white blood cell.

Fig. 4-14b, p. 62 cell wall central vacuole plasma membrane chloroplast mitochondrion nucleus 1 µm (b) Photosynthetic cell from a blade of timothy grass.

4.7 Visual Summary of Eukaryotic Cells

4.8 The Nucleus The nucleus keeps eukaryotic DNA away from potentially damaging reactions in the cytoplasm The nuclear envelope controls when DNA is accessed

The Nuclear Envelope Nuclear envelope –Two lipid bilayers pressed together as a single membrane surrounding the nucleus –Outer bilayer is continuous with the ER –Nuclear pores allow certain substances to pass through the membrane

The Nucleoplasm and Nucleolus Nucleoplasm –Viscous fluid inside the nuclear envelope, similar to cytoplasm Nucleolus –A dense region in the nucleus where subunits of ribosomes are assembled from proteins and RNA

The Chromosomes Chromatin –All DNA and its associated proteins in the nucleus Chromosome –A single DNA molecule with its attached proteins –During cell division, chromosomes condense and become visible in micrographs –Human body cells have 46 chromosomes

Chromosome Condensation

4.9 The Endomembrane System Endomembrane system –A series of interacting organelles between the nucleus and the plasma membrane –Makes lipids, enzymes, and proteins for secretion or insertion into cell membranes –Other specialized cell functions

The Endoplasmic Reticulum Endoplasmic reticulum (ER) –An extension of the nuclear envelope that forms a continuous, folded compartment Two kinds of endoplasmic reticulum –Rough ER (with ribosomes) folds polypeptides into their tertiary form –Smooth ER (no ribosomes) makes lipids, breaks down carbohydrates and lipids, detoxifies poisons

Vesicles –Small, membrane-enclosed saclike organelles that store or transport substances Peroxisomes –Vesicles containing enzymes that break down hydrogen peroxide, alcohol, and other toxins Vacuoles –Vesicles for waste disposal

Golgi Bodies and Lysosomes Golgi body –A folded membrane containing enzymes that finish polypeptides and lipids delivered by the ER –Packages finished products in vesicles that carry them to the plasma membrane or to lysosomes Lysosomes –Vesicles containing enzymes that fuse with vacuoles and digest waste materials

The Endomembrane System

4.10 Lysosome Malfunction When lysosomes do not work properly, some cellular materials are not properly recycled, which can have devastating results Different kinds of molecules are broken down by different lysosomal enzymes –One lysosomal enzyme breaks down gangliosides, a kind of lipid

Tay Sachs Disease In Tay Sachs disease, a genetic mutation alters the lysosomal enzyme that breaks down gangliosides, which accumulate in nerve cells –Affected children usually die by age five

4.11 Other Organelles Eukaryotic cells make most of their ATP in mitochondria Plastids function in storage and photosynthesis in plants and some types of algae

Mitochondria Mitochondrion –Eukaryotic organelle that makes the energy molecule ATP through aerobic respiration –Contains two membranes, forming inner and outer compartments; buildup of hydrogen ions in the outer compartment drives ATP synthesis –Has its own DNA and ribosomes –Resembles bacteria; may have evolved through endosymbiosis

Mitochondrion

Plastids –Organelles that function in photosynthesis or storage in plants and algae; includes chromoplasts, amyloplasts, and chloroplasts Chloroplasts –Plastids specialized for photosynthesis –Resemble photosynthetic bacteria; may have evolved by endosymbiosis

The Chloroplast

The Central Vacuole Central vacuole –A plant organelle that occupies 50 to 90 percent of a cell’s interior –Stores amino acids, sugars, ions, wastes, toxins –Fluid pressure keeps plant cells firm

4.12 Cell Surface Specializations A wall or other protective covering often intervenes between a cell’s plasma membrane and the surroundings

Eukaryotic Cell Walls Animal cells do not have walls, but plant cells and many protist and fungal cells do Primary cell wall –A thin, pliable wall formed by secretion of cellulose into the coating around young plant cells Secondary cell wall –A strong wall composed of lignin, formed in some plant stems and roots after maturity

Plant Cell Walls

Fig. 4-22a, p. 70

Fig. 4-22b, p. 70

Fig. 4-22c, p. 70

Plant Cuticle Cuticle –A waxy covering that protects exposed surfaces and limits water loss

Matrixes Between Animal Cells Extracellular matrix (ECM) –A nonliving, complex mixture of fibrous proteins and polysaccharides secreted by and surrounding cells; structure and function varies with the type of tissue –Example: Bone is mostly ECM, composed of collagen (fibrous protein) and hardened by mineral deposits

ECM A bone cell surrounded by extracellular matrix

Cell Junctions Cell junctions allow cells to interact with each other and the environment In plants, plasmodesmata extend through cell walls to connect the cytoplasm of two cells Animals have three types of cell junctions: tight junctions, adhering junctions, gap junctions

Cell Junctions in Animal Tissues

Key Concepts: Eukaryotic Cells Cells of protists, plants, fungi, and animals are eukaryotic; they have a nucleus and other membrane-enclosed compartments They differ in internal parts and surface specializations

4.13 The Dynamic Cytoskeleton Eukaryotic cells have an extensive and dynamic internal framework called a cytoskeleton Cytoskeleton –An interconnected system of many protein filaments – some permanent, some temporary –Parts of the cytoskeleton reinforce, organize, and move cell structures, or even a whole cell

Components of the Cytoskeleton Microtubules –Long, hollow cylinders made of tubulin –Form dynamic scaffolding for cell processes Microfilaments –Consist mainly of the globular protein actin –Make up the cell cortex Intermediate filaments –Maintain cell and tissue structures

Fig (a-c), p. 72

Fig. 4-26d, p. 72

Motor Proteins Motor proteins –Accessory proteins that move molecules through cells on tracks of microtubules and microfilaments –Energized by ATP –Example: kinesins

Motor Proteins: Kinesin

Cilia, Flagella, and False Feet Eukaryotic flagella and cilia –Whiplike structures formed from microtubules organized into arrays –Grow from a centriole which remains in the cytoplasm as a basal body Psueudopods –“False feet” used by amoebas and other eukaryotic cells to move or engulf prey

Moving Cells Flagellum of the human sperm, and pseudopods of a predatory amoeba

Fig. 4-29a, p. 73 Eukaryotic Flagella and Cilia

Fig. 4-29b, p. 73

Fig. 4-29c, p. 73

4.13 Key Concepts: A Look at the Cytoskeleton Diverse protein filaments reinforce a cell’s shape and keep its parts organized As some filaments lengthen and shorten, they move cell structures or the whole cell

Summary: Components of Prokaryotic and Eukaryotic Cells